The experiments described herein are designed to evaluate the biological role of the hepatic microsomal hydroperoxide peroxidase. Its activity will be distinguished from the other microsomal enzyme activities, in order to demonstrate which components of the MFO are required for peroxidase activity. The microsomal membrane has been solubilized using the anionic detergent, Emulgen 911. Separation of the enzymes and cytochromes has been accomplished using column chromatography (DEAE-cellulose, hydroxy-apatite, CM Sephadex and Sephadex G-75). Purification is based on specific activity and polyacrylamide electrophoresis. Recombination of these fractions in the presence of membrane phospholipids will be used to evaluate the components of the microsomal system needed for the peroxidase activity. This will allow for evaluation of the components competing or interacting in other activities in the microsomal ets. In addition, the role of the NADPH Cyt. b5 Reductase can be examined. Finally, participation of the microsomal peroxidase in maternal, placental, fetal and newborn guinea pig xenobiotic metabolism or as mechanism to protect this intricately balanced system from a strong oxidizing agent, the lipid hydroperoxides.